RFR: 8322149: ConcurrentHashMap copy constructor should not transfer from old table on presizing [v2]

[Joshua Cao]

> ConcurrentHashMap's copy constructor calls `putAll()` -> `tryPresize()` -> `transfer()`. When coming from the copy constructor, the Map is empty, so there is nothing to transfer. But `transfer()` will still copy all the empty nodes from the old table to the new table.
> 
> This patch avoids this work by only calling `tryPresize()` if the table is already initialized. If `table` is null, the initialization is deferred to `putVal()`, which calls `initTable()`.
> 
> ---
> 
> ### JMH results for testCopyConstructor
> 
> before patch:
> 
> 
> Result "org.openjdk.bench.java.util.concurrent.Maps.testCopyConstructor":
>   937395.686 ±(99.9%) 99074.324 ns/op [Average]
>   (min, avg, max) = (825732.550, 937395.686, 1072024.041), stdev = 92674.184
>   CI (99.9%): [838321.362, 1036470.010] (assumes normal distribution)
> 
> 
> after patch:
> 
> 
> Result "org.openjdk.bench.java.util.concurrent.Maps.testCopyConstructor":
>   620871.469 ±(99.9%) 59195.406 ns/op [Average]
>   (min, avg, max) = (545304.633, 620871.469, 689013.573), stdev = 55371.419
>   CI (99.9%): [561676.063, 680066.875] (assumes normal distribution)
> 
> 
> Average time is decreased by about 33%.

Joshua Cao has updated the pull request incrementally with one additional commit since the last revision:

  Cleanup benchmark

-------------

Changes:
  - all: https://git.openjdk.org/jdk/pull/17116/files
  - new: https://git.openjdk.org/jdk/pull/17116/files/93b9072d..3632649c

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=17116&range=01
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=17116&range=00-01

  Stats: 12 lines in 2 files changed: 2 ins; 7 del; 3 mod
  Patch: https://git.openjdk.org/jdk/pull/17116.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/17116/head:pull/17116

PR: https://git.openjdk.org/jdk/pull/17116